1. Technical Field
The present invention relates to an apparatus for recovering data from a data recording medium and a method thereof. More particularly, it relates to an apparatus and a method for recovering data while correcting an error in a READ signal read from the data recording medium.
2. Description of the Related Art
Typical examples of recording media for storing data, such as documents, images, and sound are a HD (Magnetic Disk), a DVD (Digital Video Disk or Digital Versatile Disk), an MO (Mageneto-optic Disk), a CD (Compact Disk), and an LD (Laser Disk), or the like. FIG. 7(a) shows an outline of a data recovery apparatus 70 for recovering data from a medium 78. A READ signal read from the medium 78 is digitized with a level determining unit 72 to be converted to a bit string composed of 0 and 1. The bit string (binary data) is sent to a decoder 74 to be decoded by using, for example, a translation table 76.
FIG. 7(b) shows an example of the structure of the level determining unit 72. A READ signal read from a medium 78 is compensated by an equalizer 88 and then input to one of two inputs of a slice circuit 80 and a slice signal generation circuit 86. A slice signal Vr generated by the slice signal generation circuit 86 is input to the other input of the slice circuit 80. The slice circuit 80 detects an intersection of the equalized signal V(t) and the slice signal Vr and then sends a detection signal to a binary data generation circuit 82. The binary data generation circuit 82 defines time cells based on the frequency of the equalized signal V(t) which is detected by a PLL (Phase Locked Loop) 84 to generate binary data.
FIG. 8 shows a method for creating a binary signal from the equalized signal V(t). Time cells are defined in a time-axis direction (t) on the basis of the frequency of the equalized signal V(t) detected by the PLL 84. Binary data is obtained by defining time cells including time T1 and T2 at the intersections of the equalized signal V(t) and the slice signal Vr as 1, and the time cells not including the intersections are defined as 0.
The precision of the creation of binary signals from the equalized signal depends on shifts in the intersections of the equalized signal V(t) and the slice signal Vr. As the shifts of the intersections become larger, the possibility that the intersections are allocated to incorrect time cells grows. When data is recovered from the medium 78, such as CD and DVD, variation of light reflectivity, interference between waveforms, and noise or the like adversely affect the equalized signal V(t). As a result, a bit shift by 1 bit often occurs.
The equalizer 88 adjusts the gain-frequency characteristics of the READ system. As shown in FIG. 9, an output of the high frequency side is mainly amplified. A dashed line shown in FIG. 9 is an output from the medium 78 and a continuous line is an output after being amplified by the equalizer 88. Although the amplification of the output performed by the equalizer 88 compensates the output of the high frequency side, noise is also amplified.
The equalizer 88 is constructed of, for example, a 7 to 17-stage transversal filter circuit. Removable media, such as DVD, require for compensation a filter circuit for each medium. When there are flaws and warping or the like in a medium such as DVD, the setting of the filter circuit may be needed to be modulated for each track or each sector. It is difficult to optimize an equalizer for each track or each sector because a great amount of calculation is required to determine the optimum setting for correction.
FIG. 10 illustrates a bit shift example when noise or characteristic degradation have affected the equalized signal. In the equalized signal V(t) shown in FIG. 10, the waveform is changed from the signal indicated at the dotted line to that indicated at the solid line. The intersection of the equalized signal V(t) and the slice signal Vr is shifted from the should-be time t to time t′. As a result, bit “1” is shifted to a time cell which is adjacent to the left side in the figure.
The slice signal Vr output from the slice signal generation circuit 86 is defined based on a peak-to-peak value of the equalized signal V(t) or the operation result of DSV (Digital Sum Variation). DSV is an accumulated value determined by adding a value of +1 when the equalized signal V(t) at the time of digitization is larger than the slice signal Vr and a value of −1 when the equalized signal V(t) is smaller than the slice signal Vr. The slicing level is set so that DSV can be zero. As shown in FIGS. 11 (a) and 11(b), when the level of the slice signal Vr is shifted to Vr′, Time T1 and T2 at the intersections are shifted to T1′ and T2′. This may cause a bit shift.
It is an object of the present invention to correct a bit shift of binary data read from a medium and further enable correction of a bit error due to mis-correction of the bit shift.